The intake and exhaust valves of an internal combustion engine are typically closed by springs and opened by cams operating on the valve stems by the application of force through some sequence of linkages which may include cam followers, rocker arms, and pushrods. Ideally, the sum of the lengths of all the components that open and close the valves should allow the system to operate at zero lash at all times, despite manufacturing variations and changing conditions of wear and thermal expansion. Zero lash with interference can prevent the valves from seating, disrupt engine operation, and subject the valve train to excessive forces, excessive wear, and deposits from incomplete combustion. Lash can cause noisy engine operation as well and component failure from fatigue.
Hydraulic lash adjusters have been in use for years to provide a mechanism for dynamically altering the length of a component in the valve train during operation to eliminate lash. The configuration of these lash adjusters includes a piston fitted in a bore for trapping a volume of fluid. The lash adjuster admits some of the volume of fluid through a check valve when a small spring is able to push the piston upwardly to take up any lash. Additionally, these lash adjusters provide for a leak path to leak fluid while the adjuster is under pressure either from interference or from normal cam lift. The designed leak rate, set by expensively precise machining of the piston-to-bore clearance, is also affected by wear, thermal expansion, and oil viscosity which varies widely with operating temperature, oil grade, contamination, dilution, or aeration.
These prior lash adjuster designs which depends on leak rate to eliminate lash contain an inherent conflict between lash adjustment rate capability with tightest clearances and cold viscous oil on the one hand, and loss of valve lift from excessive leakdown with thin oil and loose clearances on the other hand. Engine operating strategies for rapid catalyst light off on startup demand the most rapid compensation for exhaust valve expansion when the oil is cold and least able to accommodate expansion. Thus, the capabilities of the conventional lash adjuster designs are unable to adequately compensate for valve expansion which occurs during engine startup with certain engine operating strategies.